1. Field of the Invention
The present invention is related to methods for testing materials and structures, and more specifically, to a method and apparatus for nondestructive testing of objects for their mechanical behaviors.
2. Discussion of the Prior Art
Known in the art is a method to determine stresses and strains in a loaded object, wherein the object is illuminated by coherent laser radiation before and after loading in order to obtain pairs of superimposed speckle patterns. These speckle patterns are transformed into a diffraction pattern, which is then mathematically processed. Signal amplitudes obtained by this mathematical processing are used to obtain values of strains and stresses at various points of an elastically deformed object (e.g., PCT 87/07365).
The above mentioned method is deficient because it does not allow kinetics of plastic deformation process to be analyzed since it is not designed to obtain three dimensional waves of plastic deformation. The above mentioned method permits calculation of stress and distortion fields according to the detected deformation, but does not allow one to analyze the plastic flow caused by plastic deformation. Therefore, the above mentioned conventional method is suitable only for estimation of elastic deformation and is practically unsuitable for the region of plastic deformation.
Capability to analyze the dynamics of plastic deformation is extremely important for many application problems, especially for nondestructive testing of mechanical behaviors of objects under loading since a failure is always preceded by a localized plastic deformation. Inadequacy of conventional approaches to this problem has been outstanding for a long time.
Also known are methods aimed at determining contribution of various mechanisms of plastic deformation to the actual process of plastic flow of the material and its failure (Handbook on Experimental Mechanics, Ed. by A. S. Kobayashi, Prentice-Hall 1987). All these methods are based on microscopic investigations of a defective structure of deformed material in general after the load is removed. A wide range of investigation methods are used: optical and electronic microscopy, X-ray structural analysis, various types of microscopic mechanical testing, etc.
These methods are deficient since they are highly labor intensive; they require the use of sophisticated equipment as well as requiring an extremely accurate localization of the area under examination because they employ high-resolution techniques. The critical disadvantage of these methods is the fact that they are applicable only to the analysis of residual defects of the material and not applicable to the analysis of generation and movement of defects.
The most commonly used method for obtaining data on reliability of materials (See for example Collacott R. A. "Structural Integrity Monitoring", London, Chapman and Hall 1985) consists of mechanical tests of specimens under various types of loading conditions including tension, compression, bending, torsion, crack-sample test, etc. to obtain such characteristics as elastic limit, yield limit, ultimate strength, stress intensity factor; fracture toughness, ultimate fatigue, long-time strength, etc.
These tests are generally carried out utilizing specially prepared specimens of preset shape and dimensions. Numerical values of strength and plasticity characteristics thus obtained are used for strength calculations of machine parts and structural members as well as for calculations of their reliability characteristics.
This method is deficient in that a special set of mechanical characteristics and destruction criteria is used for each type of testing. These characteristics for various types of tests are not rigorously correlated to each other, and their physical interpretations are based on numerous contradictory models which often role each other out.
Another disadvantage of this method resides in problems associated with the use of data on mechanical properties of parts of mechanisms or machines obtained under laboratory conditions rather than in actual operating conditions. A machine part in actual operation or, field-use part, is generally in a more complicated stressed state than under laboratory tests. Moreover, mechanical characteristics of the material of an actual part are influenced by the surface condition as well as by the environment. Attempts to take into account these factors during mechanical tests result in complication of the test equipment and the procedure without, however, allowing an adequate description of required data under such conditions.
Finally, the mechanical characteristics obtained during the above mentioned tests are in essence averaged over the whole volume of the specimen being tested. This makes it difficult to use these characteristics for the description of properties of polycrystalline materials with special interfaces and composites consisting of areas with varied elasticity modulus and strength properties.
A common disadvantage of all existing methods for testing of plastic materials exits in the fact that they cannot record the process itself of the development of plastic deformation, and therefore are not capable of predicting the location of future failure.
In order to solve such problems as mentioned in the above, a radically new method to analyze plastic deformation is needed which would allow dynamics of deformation to be locally studied and, in particular, prediction of behavior of objects under various actual loading conditions.
The applicant of the present patent has developed a theoretical foundation for such a new method (See for example "Structural Levels of Plastic Deformation and Failure" V. E. Panin et al., Novosibirsk, Nauka, 1990). The gist of this method of analysis of mechanical behavior of an object is based on the wave theory of plastic deformation, according to which plastic deformation and subsequent failure occur as a wave process and its parameters (i.e. wavelength, amplitude and the rate of propagation) depend on properties of the material and loading conditions. Any changes in parameters of this wave process indicates the presence of changes in the structure of the material under loading. However, when the above book was published, quantitative criteria for diagnosis of these changes were not established.
The applicant of the present patent discovered the wave nature of plastic deformation, established the theoretical basis of the phenomenon and investigated it experimentally for a simple loading condition as described in "Structural Levels of Plastic Strain and Destruction" V. E. Panin et al., Novosibirsk, Nauka, 1990.
According to the proposed scheme, the object under study is illuminated by coherent laser light and photographed. The photographic plate is exposed again after a deformation has taken place without moving the plate. After the photographic plate is chemically processed, data on plastic displacement is decoded by point scanning of the speckle photography with a narrow laser beam. Parameters of the resulting Young band diffraction pattern are measured at each point, and the modulus of a displacement vector during the period between the exposures is determined from distance between adjacent bands.
Then by spatially differentiating in respect of the axes of the coordinates, components of distortion tensor, for example, the component of shear deformation, EQU .epsilon.xy=1/2 (.DELTA.u/.DELTA.y+.DELTA.v/.DELTA.x)
and rotation EQU .omega.z=1/2 (.DELTA.v/.DELTA.x-.DELTA.u/.DELTA.y)
are obtained.
Self-concordant distribution of values of plastic tensor components .epsilon.xy and .omega.xy in an object forms a relaxation wave of plastic deformation.
The idea that wave parameters obtained in the above fashion can be used for forecasting failure of a material was suggested in the book "Structural Levels of Plastic Deformation and Failure", V. E. Panin et al., Novosibirsk, Nauka, 1990. For details see this book.
However, neither general regularities concerning the evolution of plastic deformation waves based upon increase in the degree of deformation nor the relationships between deformations and subsequent failures had been studied by that time. Quantitative criteria for forecasting failures by parameters of wave patterns under various loading conditions had not been established either. Consequently, the analysis of wave patterns could not be utilized for the development of a new method of nondestructive testing of mechanical behaviors of objects under loading.
Taking into account the above mentioned drawback of the conventional technology, the purpose of the present invention is to provide a novel and improved method and apparatus concerning the nondestructive testing of mechanical behavior of an object and the criteria to diagnose the mechanical state, by obtaining three-dimensional wave patterns of plastic deformation and variation of the plastic distortion tensor either at the testing stage or under operation.